Human atria play an important role in extracellular homeostasis through release of atrial natriuretic peptide. To evaluate the relationship between plasma level of atrial natriuretic peptide (ANP) and many changes which can develop during extracorporeal circulation, we studied 16 patients undergoing, 12 cardiac operation and 4 thoracic operation. Plasma level of ANP in cardiac patients group was significantly higher and more changeable than thoracic patients group. After aortic cross clamp release, blood was filled at right atrium and right atrial pressure was rapidly increased. At the same time, plasma level of ANP was rised suddenly. Increase of ANP level was correlated (p<0.05) with the increase of total bypass time, but was not correlated statistical with aortic cross clamp time. ANP level did not fall rapidly after aortic cross clamp while both atria were completely empty. This result was explained by intraoperative hypothermia at that time, which can inactivate plasmal endopeptidase and catalytic receptors of ANP. The ANP level of atrial fibrillation group in cardiac patients were generally higher than normal sinus group, but there was no statistical correlation.